Synthesis of γ-Lactams by Formal Cycloadditions with Ketenes

One-pot synthesis of γ-lactams from ketoaziridines

The remarkable biological activities of γ-lactams have stimulated the search for efficient synthetic methods to achieve these scaffolds. In this work, we have developed a simple one-pot diastereoselective synthesis of new γ-lactams from ketoaziridines with moderate to good yields via the Horner-Wadsworth-Emmons reaction, followed by an intramolecular ester-aziridine cyclization and its opening in situ. Preliminary efforts towards an enantioselective version of this method are also reported.

Lewis Acid Catalyzed Formal (3+2)-Cycloaddition of Bicyclo[1.1.0]butanes with Ketenes

Design, synthesis and application of benzene bioisosteres have attracted a lot of attention in the past 20 years. Recently, bicyclo[2.1.1]hexanes have emerged as highly attractive bioisosteres for ortho- and meta-substituted benzenes. Herein we report a mild, scalable and transition-metal-free protocol for the construction of highly substituted bicyclo[2.1.1]hexan-2-ones through Lewis acid catalyzed (3+2)-cycloaddition of bicyclo[1.1.0]-butane ketones with disubstituted ketenes. The reaction shows high functional group tolerance as documented by the successful preparation of various 3-alkyl-3-aryl as well as 3,3-bisalkyl bicyclo[2.1.1]hexan-2-ones (26 examples, up to 89 % yield). Postfunctionalization of the exocyclic ketone moiety is also demonstrated.

Theoretical Insight into the Mechanism of Cu(I)-Catalyzed [2 + 2 + 1] Cycloaddition to β-Pyrrolinones: Azaheterocycle Formation and Assisted Dehydrogenation with Solvent MeNO2

The construction of multisubstituted β-pyrrolinones from simple starting materials remains a great challenge. Recently, a novel Cu(I)-catalyzed [2 + 2 + 1] cycloaddition reaction was developed for rapid access to fully substituted β-pyrrolinones, which are difficult to synthesize through traditional methods as this approach may involve unusual C-nucleophilic addition of enamines and umpolung of imines. Elucidating the reaction mechanism may inspire the development of new methodologies via the unusual C-nucleophilic addition of enamines and imines. However, the reaction mechanism is still unclear because none of the intermediates was observed during the reaction process. In this work, we employed theoretical and computational chemistry to investigate the possible pathway. Finally, the calculated results indicate that ketene formed by the Wolff rearrangement of α-diazo-β-ketoester reacts with enamine formed by the addition of alkynes and amine, affording the five-membered azaheterocycle, and this process involves the formation of a six-membered ring intermediate and sequential isomerization, and the further dehydrogenation needs to be assisted with solvent MeNO₂.

1,4-Aryl migration in ketene-derived enolates by a polar-radical-crossover cascade

The arylation of carboxylic acid derivatives via Smiles rearrangement has gained great interest in recent years. Both radical and ionic approaches, as well as radical-polar crossover concepts, have been developed. In contrast, a reversed polar-radical crossover approach remains underexplored. Here we report a simple, efficient and scalable method for the preparation of sterically hindered and valuable α-quaternary amides via a polar-radical crossover-enolate oxidation-aryl migration pathway. A variety of easily accessible N-alkyl and N-arylsulfonamides are reacted with disubstituted ketenes to give the corresponding amide enolates, which undergo upon single electron transfer oxidation, a 1,4-aryl migration, desulfonylation, hydrogen atom transfer cascade to provide α-quaternary amides in good to excellent yields. Various mono- and di-substituted heteroatom-containing and polycyclic arenes engage in the aryl migration reaction. Functional group tolerance is excellent and substrates as well as reagents are readily available rendering the method broadly applicable.

The α-arylation of amides via aryl migration has attracted considerable interest in recent years. Here, the authors report a method for the preparation of bulky α-quaternary amides via a polar-radical crossover enolate oxidation-aryl migration cascade.

Microwave-accelerated and efficient synthesis of structurally diverse N-(2,2-diphenylvinyl)-β-oxoamides

N-(2,2-Diphenylvinyl)-β-oxoamides are both the structural moiety of biologically active compounds and important synthetic intermediates. Structurally diverse N-(2,2-diphenylvinyl)-β-oxoamides are prepared efficiently from 2-diazo-1,3-dicarbonyl compounds and N-alkyl-2,2-diphenylaziridines via an electrophilic ring opening reaction under two different reaction conditions of reflux and microwave irradiation. 2-Diazo-1,3-dicarbonyl compounds undergo the Wolff rearrangement under heating to generate α-oxoketenes, which electrophilically react with N-alkylaziridines to directly produce structurally diverse N-(2,2-diphenylvinyl)-β-oxoamides in good to excellent yields under microwave irradiation. Microwave irradiation accelerates the reaction obviously and efficiently. Both 2-diazo-1,3-diketones and alkyl 2-diazo-3-oxoalkanoates work well. The reaction is catalyst-free and highly atom economical, involves only loss of nitrogen and does not require additives. The products are useful synthons for the convenient preparation of multisubstituted β-lactam derivatives.